In a fuel cell, a catalyst layer (CL) is located between the proton exchange membrane (PEM) and the gas diffusion layer (GDL). Protons transfer between the CL and the PEM, and electrons transfer between the catalyst layer and the GDL. All these elements require good interfacial contact. In a PEM fuel cell, the CLs are where the electrochemical reactions occur for electric power generation. For example, for H2/air (O2) PEM fuel cells, the reactions occurring at the anode and cathode catalyst layers are as follows:Anode: H2→2H++2e−  (1)Cathode: O2+4H++4e−→H2O  (2)
For both reactions to occur, a three-phase boundary is required where the reactant gas, protons, and electrons react at the catalyst surface. The CLs should be able to facilitate transport of protons, electrons, and gases to the catalytic sites. Under normal PEM fuel cell operating conditions (≦80° C.), the reactants are gaseous phase H2 and O2 (from air), and the product is water, primarily in the liquid phase. Water removal is a key factor affecting catalyst layer performance. The presence of excess water in the catalyst layer can block gas transport, leading to reduced mass transfer and decreased fuel cell performance. On the other hand, a lack of water results in decreased proton conductivity of the membrane and the ionomer in the catalyst layers, leading to decreased fuel cell performance. Because the cathode side is the limiting factor in PEM fuel cells (slow O2 reduction reaction kinetics and significant water management issues), the majority of studies are focused on the cathode CL. The basic requirements for a CL include:                a large number of three-phase boundary sites;        efficient transport of protons from the anode catalyst layer to the cathode catalyst layer;        facile transport of reactant gases to the catalyst surface;        efficient water management in the catalyst layers; and        good electronic current passage between the reaction sites and the current collector.        
The microstructure and composition of the CL in PEM fuel cells play a key role in determining the electrochemical reaction rate and power output of the system. Other factors, such as the preparation and treatment methods (temperature, pressure), can also affect catalyst layer performance. Therefore, optimization of the catalyst layer with respect to all these factors is a major goal in fuel cell development.
An optimal catalyst layer design is required to improve catalyst (platinum or platinum alloys etc.) utilization and thereby reduce catalyst loading and fuel cell cost. Currently a thin-film CL technique remains the most commonly used method in PEMFCs. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [Wilson, M. S., and Gottesfeld, S. Thin film catalyst layers for polymer electrolyte fuel cell electrodes. Journal of Applied Electrochemistry 1992; 22:1-7], Ballard, and Johnson-Matthey [Ralph, T. R., Hards, G. A., Keating, J. E., Campbell, S. A., Wilkinson, D. P., Davis, M., St-Pierre, J., and Johenson, C. Low cost electrodes for proton exchange membrane fuel cells. Journal of the Electrochemical Society 1997; 144:3845-3857]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is rather hydrophilic perfluorosulfonic acid ionomer known under the name of Nafion (trademark), which also provides proton conductive paths for the electrochemical reactions.
In practice, the catalyst used in the thin-layer CLs for both anode and cathode is carbon-supported Pt catalyst (Pt/C) or Pt alloy, such as PtRu/C, although unsupported catalysts can be used. In terms of the overall electrode structure, an electrode with a thin CL generally contains three layers: carbon backing (paper), a thin carbon/PTFE microporous gas diffusion layer, and a thin-film ionomer/catalyst layer.
In general, higher Pt loading leads to better performance, but it also results in higher cost, which is one of the key factors hindering PEM fuel cell commercialization. In high Pt loading structures 40-60% of Pt is unutilized. Careful engineering, optimal design of the catalyst layer structure and microstructure would allow reducing catalyst loading by increasing its utilization.
Therefore, one of the major goals in PEM fuel cell development is to reduce Pt loading without compromising fuel cell performance and durability. At the present stage of technology, optimal Pt loading in terms of both practical fuel cell performance and durability is about 0.3 mg/cm2.
There are two main types of thin-film catalyst layers: catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane CCM, in which the CL is directly coated on the proton exchange membrane. The most obvious advantage of the CCM is better contact between the CL and the membrane, which can improve the ionic connection and produce a nonporous substrate, resulting in less isolated catalysts. An early conventional CCM based on a Pt/perfluorosulfonic acid mixture was developed at Los Alamos National Laboratory in the United States [Wilson, M. S., and Gottesfeld, S. Thin film catalyst layers for polymer electrolyte fuel cell electrodes. Journal of Applied Electrochemistry 1992; 22:1-7]. The authors used a so-called decal method to prepare a thin-film CCM in which the catalyst ink was first applied to a Teflon blank and then transferred to the membrane by hot pressing.
Based on the nature of catalyst ink and its application method, several thin-film CL fabrication techniques have been developed. Currently, screen printing and spray coating have become standard methods for conventional catalyst layer fabrication. Inkjet printing demonstrated the capacity to control ink volume for low catalyst loading however fuel cell testing on the fabricated CLs did not show any performance advantages [Towne, S., Viswanathan, V., Holbery, J., and Rieke, P. Fabrication of polymer electrolyte membrane fuel cell MEAs utilizing inkjet print technology. Journal of Power Sources 2007; 171:575-584]. The maximum power densities achieved with a cathode catalyst loading of 0.20 mg Pt/cm2 is 155 mW/cm2.
Numerous efforts have been made to improve existing thin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electrode performance. In thin-film ink-based CL fabrication, the most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as a perfluorosulfonic acid ionomer and then to apply this ink on a porous support or membrane using various methods (U.S. Pat. No. 5,234,777). In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction occurs only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access.
For the technique that applies the ink directly applied to the membrane, the membrane has to be converted to Na+ or K+ form to increase its robustness and thermoplasticity.
Another substantial disadvantage of ink-based CL fabrication relates to poor capacity to control and optimize micro-, meso- and macro-structure of CL during its formation on a support or membrane and at the hot-pressing step. The features of ink-based catalyst layers namely wetting properties, porosity, ionic (proton) and electronic conductivity affecting fuel cell performance through water transport, electrochemically active surface area, and gas transport are predetermined at the initial stage of the ink formation and entirely depend on the ink composition. Optimization of ink composition and content of the main components such as ionomer (a perfluorosulfonic acid), catalyst (Pt or Pt alloys), support (carbon), and pore-former allowed lowering the catalyst loading but not sufficiently to contribute to PEMFC commercialization.
There is a need of an in-situ CL layer fabrication method that enables control, optimized design, morphology, and structure of the catalyst layer during its formation (deposition) in order to have more opportunities for reducing catalyst (Pt or Pt-alloys) loading and increasing catalyst utilization without sacrificing electrode performance.
Optimization of an ink-based CL deposited onto a gas diffusion layer has been carried out through modeling and simulation [Wang, Q., Eikerling, M., Song, D., Liu, Z., Navessin, T., Xie, Z., and Holdcroft, S., Functionally graded cathode catalyst layers for polymer electrolyte fuel cells, Journal of the Electrochemical Society 2004; 151:A950-A957] and demonstrated enhanced performance of PEMFC with functionally 1-dimensional graded cathode catalyst layer. There are contradictory results in the literature related to optimizing CL performance, due to the complexity induced by proton and electron conduction, reactant and product mass transport, as well as electrochemical reactions within the CL. Modeling has been performed for base-case conditions and physical properties typical to relatively high catalyst loaded (0.42 mg Pt/cm2) CLs produced by brushing, printing or spray coating. There is no indication in literature related to simulation of ultra-low loaded catalyst layers deposited by in-situ CL layer deposition methods.
Additionally, the ink prepared by mixing a carbon supported platinum with Nafion and possibly other surfactants and then spraying limits the achievable film thickness to 1 μm. A process capable of attaching the platinum to the carbon with and without Nafion would allow for the formation of hereto-unachievable structures and thinner CLs.
An apparatus for manufacturing a CL structure with 1-dimensional grading by ink-jet printing is disclosed in patent application US 2005/0098101. The method and apparatus enable to form CL having compositionally graded depth only through multi-step process building up a multiple layer material. The minimum thickness of a single ink spray coated layer is about 1 micron, which makes this method not applicable for fabrication of thin graded CLs with ultra-low catalyst loading. Another apparatus for applying nano-sized layers according to a Reaction Spray Deposition Technology (RSDT) is described in applicant's published PCT application no. WO 2007/045089, the disclosure of which is incorporated herein by reference.
Another approach to reducing the catalyst loading while increasing CL durability relates to application of unsupported catalyst layer on PEM. Carbon suffers from weak corrosion resistance in fuel cell operating conditions. The elimination of the carbon support would allow to improve CL durability and to lower the catalyst loading. However, current methods for fabrication of unsupported CLs have substantial disadvantages hindering commercial application of such CL material. The final microstructure is extremely important for unsupported catalyst as the need to avoid reactant inaccessible catalyst sites is increased in the absence of a supporting medium. The application of the modified thin film method, despite its relatively higher Pt utilization, to micro-PEMFC applications has proven ineffective due to relatively higher Pt loadings. Although electrocatalysts fabricated by the electrodeposition method achieved the highest Pt utilization, the application of this method to large-scale manufacturing is doubtful due to concerns regarding its scalability. The advantage of the sputter method is its ability to deposit Pt directly onto various components of the membrane electrolyte assembly (MEA) with ultra-low-Pt-loadings. However, the low Pt utilization, non-controlled porosity and poor substrate adherence of the Pt remain challenges. Other methods, such as dual IBAD method, electro-spray technique and Pt sol methods, exhibited relatively lower Pt loadings and higher Pt utilization. However, these methods require further research to evaluate their capabilities and improve their reproducibility.
Thus, replacement of traditional carbon supported CL in PEMFC requires development of an efficient unsupported catalyst with good adherence to PEM.
PEMFCs function at various operating conditions (relative humidity, reactant gases, temperature, current density) depending on the end user fuel cell application. A majority of studies are devoted to development of novel catalyst layers demonstrating improved performance at temperatures of 80° C. and humidity 100%. It is presumed that this catalyst material will show the same advantages under other operating conditions. This assumption is not always valid because a change of any operating condition causes appropriate amendment to requirements for a CL and needs optimization of its design, structure and composition.
The known approaches do not provide PEMFC developers with alternative catalytic materials adjusted and optimized to specific operation conditions.
While continuous progress is being made with PEMFCs, there is still a need for developments offering a relatively high fuel cell performance in terms of voltage and power density (W/cm2) at a minimum possible catalyst loading to reduce the cost of the catalyst.